Combined vapour and particulate filter

ABSTRACT

With reference to FIGS. ( 1  and  2 ), a personal respirator ( 1 ) has a face mask ( 2 ) and is provided with a filter unit ( 3 ) comprising a canister ( 4 ) housing contiguous layers ( 5 ) of novel filter material ( 6 ). The filter material ( 6 ) is suitable for both particulate and vapour filtration, and comprises a composite of first and second groups ( 10, 11 ) of chopped fibres, the fibres of the first group ( 10 ) having a relatively large diameter (about 7×10 −6 m) and the second group ( 11 ) having a relatively small diameter (about 0.5×10 −6 m).

[0001] Filter materials made of several different fibre types, including fibres with different diameter dimensions are already known in the art. Database WPI Section Ch, Week 198733 Derwent Publications Ltd, London reference AN 1987-231919 discloses a filter material comprising 20-60% active carbon fibre with a diameter less than 15 ×10⁻⁶ m; 10-20% ultra thin glass fibre with a diameter of 0.2-0.5×10⁻⁶ m; and 20-70% cellulose fibre. Database WPI Section Ch, Week 198506 Derwent Publications Ltd, London reference AN 1985-034894 discloses a filter material comprising 20% fibres with a diameter 0.1-3×10⁻⁶ m eg K titanate fibre; 20% fibres with a diameter 5-15×10⁻⁶ m eg polyethylene terephthalate; and 20% fibres with a diameter 20-50×10⁻⁶ m eg polyethylene terephthalate. U.S. Pat. No. 4,765,812 discloses a filter material comprising 10-20% of a binder material, and 80-90% of a fibre mixture wherein the fibre mixture comprises 75-95% fibres with a diameter greater than or equal to 20×10⁻⁶ m and 5-25% fibres with a diameter less than 3×10⁻⁶ m.

[0002] The various aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

[0003]FIG. 1 is a side view of a respirator provided with a filter unit,

[0004]FIG. 2 is a cross-section of filter material employed by the filter unit,

[0005]FIG. 3 is a flow chart illustrating manufacturing steps used in producing the filter material and

[0006]FIGS. 4, 5 and 6 illustrate various forms of filter materials.

[0007]FIG. 1 shows a personal respirator 1 of the form disclosed by GB 2,080,120 (Secretary of State for Defence), having a face mask 2, a filter unit 3 and a canister 4.

[0008] By the current invention, canister 4 houses contiguous layers 5 of filter material 6 according to the invention (FIG. 2).

[0009] Filter material 6 is suitable for both particulate and vapour filtration, and comprises a composite of first and second groups 10, 11 of chopped fibres, the fibres of the first group 10 having a relatively large diameter (about 7.0×10⁻⁶ m) and the fibres of the second group 11 having a relatively small diameter (about 0.5×10⁻⁶ m).

[0010] The preferred range of fibre diameter is:

[0011] First group 6.0 to 14×10⁻⁶ m

[0012] Second group 0.1 to 1.0×10⁻⁶ m

[0013] Preferred length of fibres:

[0014] First group 1.0 mm to 6.0 mm

[0015] Second group 100.0×10⁻⁶ m to 200.0×10⁻⁶ m

[0016] First group fibres 10 of this example comprises Polyacrylonitrile (PAN) or pitch carbon fibres which have been activated by conventional steam or CO₂ activation methods so as to render them porous whereby they are capable of adsorbing high boiling point (say >50° C.) vapours. These fibres have also been chemically treated with impregnants so as to render them capable of adsorbing low (e.g. <50° C.) boiling point vapours. The fibres 10, which may be regarded as macrofibres, make up approximately 70% (by mass) of the composite material. The range of fibre mix may vary, with the fibres 10 of the first group ranging from 60% to 90% (by mass) and fibres 11 of the second group making up the remainder of the mass.

[0017] Fibres 11 of the second group, which may be regarded as microfibres, comprise vapour grown carbon (or glass) fibres.

[0018] The first group fibres filter vapours and the second group fibres filter particulates. By combining the two groups of fibres a combined vapour and particulate filter material results.

[0019] The dual role of the filter material results in a reduction in weight (when compared with conventional two filter units), as well as a reduction in breathing resistance, due to a lesser requirement for total filtration media.

[0020]FIG. 3 illustrates how the filter material is produced.

[0021] The desired portions of first and second groups of fibres are mixed in a mixing vessel 15 together with water, a soluble binder, such as sodium carboxymethyl cellulose or acrylic base binder and a viscosity modifier such as glycerol, which aids dispersion and ensures a substantially uniform mixture. The mixture is then passed to a sheet former 16, of the type used in the paper industry to make test samples. Water slouble binder (such as mentioned above) can be added to ensure that the subsequent composite has good mechanical properties.

[0022] If binder is added, the material is heated to 130° C. to 150° C. to cure the binder for 10-20 minutes.

[0023] Use of the former 16 results in composite filter material of mat form which is subsequently cut to size for incorporation as layers into the filter canister 4 of FIG. 1, using cutter 17.

[0024] The composite material can be produced in mats of flat sheet form, such as rectangles (FIG. 4) or discs (FIG. 5). Alternatively, the mats may be pleated (FIG. 6). The finished shapes are produced so as to allow ease of integration in the respirator system.

EXAMPLES 1.Particulate Removal

[0025] A 25g activated pitch carbon fibre mat has been produced that can remove 99.5% of NaCl particles (mean particle diameter 0.6×10⁻⁶ m) when tested at a face velocity of 30 cc/min. The associated pressure drop is low, 0.9 mm H2O.

2.Vapour Removal

[0026] Activated pitch carbon fibres tested against hexane (concentration 4000 mg/m3@ 11 /min) in dry conditions (<5% relative humidity), and dry sample. Weight of carbon fibres is 0.8 g in a 2.5 cm brass sample tube. One of the activated carbon fibres did not display any hexane breakthrough until 63 minutes. 

1. Filter material suitable for both particulate and vapour filtration comprising a composite of first and second groups of fibres, the fibres of the first group having a relatively large diameter and the fibres of the second group having a relatively small diameter.
 2. Filter material as claimed in claim 1, wherein the fibres of the first group have a range of 6.0 to 14.0×10⁻⁶ m diameter.
 3. Filter material as claimed in claim 1 or 2, wherein the fibres of the second group have a range of 0.1 to 1.0×10⁻⁶ m in diameter.
 4. Filter material as claimed in claim 1, 2 or 3 wherein the fibres of the first group have a range of 1.0 mm to 6.0 mm in length.
 5. Filter material as claimed in any one of claims 1 to 4, wherein the fibres of the second group have a range of 100.0 to 200.0×10⁻⁶ m in length.
 6. Filter material as claimed in any one of claims 1 to 3, wherein the fibres of the first group are about 7×10⁻⁶ m in diameter.
 7. Filter material as claimed in any one of claims 1 to 6, wherein the fibres of the second group are about 0.5×10⁻⁶ m in diameter.
 8. Filter material as claimed in any one of claims 1 to 7, wherein the fibres of the first group range from 60% to 90% by mass.
 9. Filter material as claimed in claim 6, wherein the fibres of the first group comprise approximately 70% by mass.
 10. Filter material as claimed in any one of claims 1 to 9, wherein the fibres of at least the first group comprise carbon fibres.
 11. Filter material as claimed in any one of claims 1 to 10, wherein the fibres of the second group comprise glass fibres.
 12. filter material as claimed in any one of claims 1 to 11, wherein the fibres of the first group have been treated so as to render them capable of adsorbing both high and low boiling point vapours.
 13. Filter material as claimed in claim 12, wherein the high boiling point vapours are greater than 50° C. and the low boiling point vapours are less than 60° C.
 14. A method of producing filter material of composite form suitable for both particulate and vapour filtration, comprising mixing together with binder, first and second groups of fibres, the fibres of the first group having a relatively large diameter and the fibres of the second group having a relatively small diameter.
 15. The method as claimed in claim 14, wherein the material is produced as sheets, subsequently cut to size for incorporation as layers in a respirator system.
 16. A respirator provided with a filter unit incorporating a filter material of any one of claims 1 to
 15. 